Inorg. Chem. 2008, 47, 2264-2266
Diruthenium(II,III) Bis(tetramethyl-1,3-benzenedipropionate) as a Novel Catalyst for tert-Butyl Hydroperoxide Oxygenation Julia E. Barker† and Tong Ren*,‡ Department of Chemistry, UniVersity of Miami, Coral Gables, Florida 33146, and Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907 Received January 9, 2008
The reaction of Ru2(OAc)4Cl with 2.2 equiv of H2esp (esp ) R,R,R′,R′-tetramethyl-1,3-benzenedipropionate) resulted in a new compound, Ru2(esp)2Cl (1), that is soluble in organic media. 1 is an active catalyst for the oxygenation of organic sulfides by tertbutyl hydroperoxide (TBHP) in both an acetonitrile solution or neat (solvent-free) conditions. Solvent-free reactions display the quantitative utility of TBHP and hence excellent chemical selectivity for sulfoxide formation.
The catalyzed oxygenation of organic sulfides is an important transformation for medicinal chemistry,1 petroleum desulfuridization,2 and nerve agent detoxification.3 For the last two applications, the reaction rates, nature of both the oxidants and catalysts, and the chemical selectivity between sulfoxide and sulfone products are highly relevant issues. Hydrogen peroxide and tert-butyl hydroperoxide (TBHP) are among the desired oxidants because of both their low costs and environmentally friendly nature.4 Hydrogen peroxide is the most studied5 and can be activated with a variety of transition-metal-based homogeneous catalysts. In comparison, tert-butyl hydroperoxide is difficult to activate and is less studied.6 The earliest example of catalytic oxidation by TBHP was reported by Kuhnen,7 in which thioanisole was converted to methyl phenyl sulfone quantitatively in the presence of molybdenyl acetylacetonate. Recently reported efficient catalysts for TBHP include MeReO(mtp)PPh3 [mtp * To whom correspondence should be addressed. E-mail:
[email protected]. Tel. (765) 494-5466. Fax (765) 494-0239. † University of Miami. ‡ Purdue University. (1) (a) Carreno, M. C. Chem. ReV 1995, 95, 1717. (b) Fernandez, I.; Khair, N. Chem. ReV. 2003, 103, 3651. (2) (a) Babich, I. V.; Moulijn, J. A. Fuel 2003, 82, 607. (b) Collins, F. M.; Lucy, A. R.; Sharp, C. J. Mol. Catal. A 1997, 117, 397. (3) (a) Yang, Y.-C. Acc. Chem. Res. 1999, 32, 109. (b) Wagner, G. W.; Yang, Y. C. Ind. Eng. Chem. Res. 2002, 41, 1925. (4) Sheldon, R. A.; Arends, I.; Hanefeld, U. Green Chemistry and Catalysis; Wiley-VCH: Weinheim, Germany, 2007. (5) Lane, B. S.; Burgess, K. Chem. ReV. 2003, 103, 2457. (6) (a) Hill, C. L.; Prossermccartha, C. M. Coord. Chem. ReV. 1995, 143, 407. (b) Romao, C. C.; Kuhn, F. E.; Herrmann, W. A. Chem. ReV. 1997, 97, 3197. (c) Kozhevnikov, I. V. Chem. ReV. 1998, 98, 171. (7) Kuhnen, L. Angew. Chem., Int. Ed. 1966, 5, 893.
2264 Inorganic Chemistry, Vol. 47, No. 7, 2008
Scheme 1. Preparation of 1a
a Conditions: Ru2(OAc)4Cl, 2.2 equiv of H2esp, H2O/CH3OH (v/v, 1:2); 3 h of reflux.
) 2-(mercaptomethyl)thiophenol)] by Espenson et al.8 and (Me3TACN)MnX3 (Me3ATCN ) 1,4,7-N,N′,N′′-trimethyl1,4,7-triazacyclononane and X ) Cl- and Br-) by us.9 In both cases, the optimal turnover frequencies (TOFs) are about 200 h-1. Although efficient, both the Re and Mn catalysts invoked elaborately designed ligands and are hence expensive. The feasibility of using dimetallic paddlewheel species to activate TBHP has been demonstrated in allylic and benzylic oxidation by TBHP in the presence of dirhodium(II) caprolactamate [Rh2(cap)4] by Doyle and co-workers,10,11 although Rh is also quite expensive. Encouraged by Doyle et al.’s success, we have been exploring the use of diruthenium species as the TBHP activators12 and reported herein are the development of a novel compound Ru2(esp)2Cl (1; Scheme 1) and the preliminary assessment of its capacity in promoting sulfide oxygenation by TBHP. 1 was prepared in excellent yield (88%) by refluxing Ru2(OAc)4Cl with 2.2 equiv of H2esp (esp ) R,R,R′,R′tetramethyl-1,3-benzenedipropionate)13 in H2O/CH3OH (1: 2).14 Although 1 cannot be characterized by NMR because (8) Wang, Y.; Lente, G.; Espenson, J. H. Inorg. Chem. 2002, 41, 1272. (9) Barker, J. E.; Ren, T. Tetrahedron Lett. 2005, 46, 6805. (10) Catino, A. J.; Forslund, R. E.; Doyle, M. P. J. Am. Chem. Soc. 2004, 126, 13622. (11) Catino, A. J.; Nichols, J. M.; Choi, H.; Gottipamula, S.; Doyle, M. P. Org. Lett. 2005, 7, 5167. (12) Barker, J. E. Ph.D. Dissertation, University of Miami, Coral Gables, FL, 2006. (13) Espino, C. G.; Fiori, K. W.; Kim, M.; Du Bois, J. J. Am. Chem. Soc. 2004, 126, 15378.
10.1021/ic800035w CCC: $40.75
2008 American Chemical Society Published on Web 03/14/2008
COMMUNICATION Scheme 2. Oxygenation of Organic Sulfides
Figure 1. Structural plot of 1. H atoms were omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru1-Ru2, 2.2768(6); Ru1-Cl1, 2.4852(14); Ru1-O1, 2.034(2); Ru1-O3, 2.028(2); Ru2-O2, 2.021(2); Ru2-O4, 2.009(2); Ru2-O5, 2.342(3); Ru2-Ru1-Cl1, 177.64(4); Ru1-Ru2-O5, 178.81(9); Ru-Ru′-O(av), 89.43(7). There is a crystallographic mirror that contains the Ru2-Ru1-Cl vector and bisects both esp ligands.
of its paramagnetic nature (S ) 3/2), its identity has been confirmed by both fast atom bombardment mass spectrometry (FAB-MS) and combustion analyses. Its molecular structure was determined through a single-crystal X-ray diffraction study,15 and the structural plot is shown in Figure 1. It is clear from Figure 1 that the esp ligand coordinates to the Ru2 core as a tetradentate chelate, and the overall geometric feature of the paddlewheel core is very similar to those of Rh2(esp)213 and Re2(esp)2Cl2.16 One of the two axial positions is occupied by the chloro ligand, a feature common to all Ru2(O2CR)4Cl-type compounds,17 while the other is occupied by a water molecule. The same axial ligation pattern was observed for Ru2(O2CCMe3)4Cl.18 The Ru-Ru distance in 1 [2.2768(6) Å] is identical with that in Ru2(O2CCMe3)4Cl [2.274(2) Å] within the experimental error, and both are within the range of the Ru-Ru distance in Ru2(O2CR)4Xtype compounds, 2.248–2.292 Å.19 Successful preparation of 1 was followed by the examination of its activity in facilitating TBHP oxygenation of organic sulfides. Organic sulfides studied were methyl phenyl sulfide (MPS), diphenyl sulfide (PPS), ethyl phenyl sulfide (EPS), and 2-chloroethyl phenyl sulfide (CEPS), which are shown in Scheme 2 along with the oxygenation reaction of (14) Ru2(OAc)4Cl (200 mg, 0.422 mmol) and H2esp ligands (259 mg, 0.929 mmol) were refluxed in a mixture of H2O (20 mL) and MeOH (30 mL) for 3 h. Upon the removal of most of MeOH, the precipitate formed was collected by filtration and rinsed with water. The crude product was purified on a short silica gel column with EtOAc/hexanes (3:1) to yield a golden burgundy solid (294 mg, 88.0% based on ruthenium). For the purpose of analysis, the product was recrystallized from CH2Cl2 and dried under vacuum for 2 days. FAB-MS (m/e, based on 101Ru): 756 [M+ - Cl]. Anal. for Ru2(esp)2Cl · 0.5CH2Cl2. Found (calcd): C, 46.60 (46.65); H, 5.37 (5.42). (15) X-ray diffraction data for crystal 1 were collected on a Bruker SMART1000 CCD diffractometer using Mo KR at 300 K. Crystal data for Ru2(esp)2Cl · 2CH2Cl2: C34H44Cl5O9Ru2, fw ) 976.08, orthorhombic, Pnma, a ) 16.9347(7) Å, b ) 21.662(1) Å, c ) 11.2152(5) Å, V ) 4114.3(3) Å3, Z ) 4, Dcalcd ) 1.576 g cm-3. Of 20 916 reflections measured, 3729 were unique (Rint ) 0.064). Least-squares refinement based on 3729 reflections with I g 2σ(I) and 238 parameters led to convergence with final R1 ) 0.037 and wR2 ) 0.082. (16) Dequeant, M. Q.; Fanwick, P. E.; Ren, T. Inorg. Chim. Acta 2006, 359, 4191. (17) Cotton, F. A., Murillo, C. A., Walton, R. A., Eds. Multiple Bonds between Metal Atoms; Springer Science and Business Media, Inc.: New York, 2005. (18) Barral, M. C.; Jimenez-Aparicio, R.; Priego, J. L.; Royer, E. C.; Saucedo, M. J.; Urbanos, F. A.; Amador, U. Dalton Trans. 1995, 2183. (19) Aquino, M. A. S. Coord. Chem. ReV. 1998, 170, 141.
Table 1. Catalytic Oxygenation of Organic Sulfides entry
RSR′
t/h RSR′ %
RS(O) R′ %
RS(O)2 R′ %
“other”
TOFa/h-1
1 MPS 1 82 15 3 0 20 2 4 43 52 5 0 16 3 12 2 85 13 0 9 4 24 99 93
0 0 0
